1. Field of the Invention
This invention relates to a process for the conditioning of plastic materials and plastic articles which are to be metal plated through use of conventional electroless and electroplating techniques. More particularly, this invention relates to an improved process for surface conditioning of such plastic materials and plastic articles so as to provide them with prolonged hydrophilic surfaces. This conditioning permits electroless and electroplating of said materials and articles to provide continuous metal plated surfaces which possess superior adhesive properties as compared to metal surfaces plated onto untreated surfaces. This invention also relates to products produced by the process of this invention.
2. Description of the Prior Art
In recent years, electroplating and metallic coating of plastics has become of ever increasing importance. In order to manufacture plastics having firmly bonded metal coatings, the plastic must be subjected to a pretreatment step, during which the surfaces of the plastic are treated to increase adhesion characteristics of metal surfaces coated thereon. Heretofore, various methods have been employed in such a pretreatment step. For example, in one such prior art method, the surfaces of the plastic are treated with a mixture of chromium trioxide, sulfuric acid and water, or a mixture of such ingredients and phosphoric acid. While relatively effective, this method suffers a number of disadvantages. For example, the efficiency of this method is very much dependent on the type of plastic to be treated; and accordingly it is mainly confined to use in the pretreatment of a narrow class of plastics. Thus, the process does not have universal applicablility for a wide range of assorted plastics.
Another disadvantage of this known method is that the relative amounts of components in the chromium trioxide containing pre-treatment solution must be maintained within a pre-determined and rather narrow range of concentrations. Only deviations of a few percent from these optimum amounts are permissible. Otherwise, on subsequent chemical deposition of the metal, the plastic surface is not completely coated by the metal and/or the entire coating does not possess sufficiently high peel strength. In addition, in this known method, the pretreatment solution becomes useless as soon as the concentration levels of the degradation products of the plastic and of the trivalent chromium compounds in the solution reach a certain level. Thus, constant analytical supervision and dosing of the chemicals consumed and monitoring of the degradation products produced during the conduct of this known pretreatment method is required. This continual control of the process considerably complicates its conduct.
Still another disadvantage of this known pretreatment process results from difficulties associated with the disposal of the used composition. To eliminate the used pretreatment composition, it is necessary to reduce the hexavalent chromium compounds and then to neutralize the reduction product. In the neutralization procedure, large amounts of highly voluminous chromium hydroxide are formed, the removal of which considerably encumbers the disposal of the used composition. Thus, the disposal of the used pretreatment composition also complicates the conduct of the process and requires considerable expenditures of time, capital, technical apparatus and plants.
Lastly, the pretreatment composition of this known process is very corrosive, and extensive washings with water are required to completely remove it from the plastic surface being pretreated. The result is a further increase in process time and expense.
Another prior art process for pretreating plastic surfaces to enhance the peel strength of metal surfaces electroless or electroplated thereon is disclosed in U.S. Pat. No. 4,039,714. In this process, the surfaces of the plastic to be electroplated are pretreated by exposing them to an atmosphere containing sulfur trioxide. This known pretreatment process also suffers from several inherent disadvantages. For example, sulfur trioxide is an extremely toxic substance, which reacts with atmospheric moisture to form sulfuric acid which is extremely corrosive and dangerous. Thus, when using sulfur trioxide extreme care must be taken to prevent contact with human skin. Furthermore, sulfur trioxide reacts violently with certain organic materials producing heat, which results in an additional hazard to users of the process. The dangerous nature of sulfur trioxide results in an increase in the time, cost and equipment required to use this process.
U.S. Pat. No. 4,422,907 discloses an improvement in processes for pre-treatment of plastic surfaces for electroplating purposes. In the process of that patent, the surfaces of the plastic material are exposed to ozone and followed by washing with an aqueous alkali solution. While the process of this patent represents a significant advancement over prior art pretreatment processes, it suffers from one disadvantage which impacts on the capability of this process to be practically and commercially operative. More specifically, this disadvantage is that conditioning of the treated surface dissipates with time. If the surface of the plastic is exposed to air after alkali conditioning and prior to treatment with the electroless plating catalyst for short periods of time for reasons of process timing, process problems and the like, the quality of the metal plate electrodeposited onto the surface is impaired.